1. Field of the Invention
This invention relates generally to surgical instruments, and more specifically to surgical instruments which contact tissue and require traction with the tissue to inhibit migration of the instrument.
2. Discussion of the Prior Art
Most surgical instruments are intended to contact tissue, but for some instruments the traction developed between the instrument and the tissue is of particular importance. Instruments such as clips, clamps, retractors, stabilizers, and spreaders, for example, are intended to contact tissue and perform some mechanical function on the tissue. In these cases, the ability of the instrument to grip the tissue contacted is of concern. For example, when a clip is applied to a blood vessel with the intent of occluding that vessel, the occlusion is intended to occur at a predetermined location along the vessel. Although little force may be required to pinch and occlude the vessel, there may be a tendency for the clip to slide either axially or laterally along the vessel. Often this results from the back pressure of the blood in the vessel. If the clip slides radially of the vessel, it may fall off the vessel, leading to unintended blood flow. If the clip slides axially along the vessel, it will leave the predetermined location where the occlusion was intended.
The sliding of instruments relative to tissue is complicated by the fact that the tissue is typically covered with a body fluid, such as blood. As a consequence, the coefficient of friction between the tissue and the instrument tends to be relatively low.
In the past, clips and clamps have been provided with soft jaw inserts in order to reduce trauma to the conduit being occluded. For the most part, these inserts have been formed of a compliant material such as foam, and provided with a generally flat surface. The traction tending to hold the clip or clamp in place has been dictated by the well known formula for friction: F=xcexcN, where F is the friction force resisting lateral movement, N is the normal force applied perpendicular to friction force, and xcexc is the coefficient of friction between the two surfaces.
In accordance with this formula, attempts have been made to increase the factor xcexc by providing inserts which have higher coefficients of friction with tissue. In spite of these efforts, traction has still been a problem since these coefficients cannot be increased significantly without damaging the vessel or other conduit being occluded.
Individual fibers in the form of loops have been applied to the inserts to improve traction. The traction in this case has relied, at least in part, on a mechanical interlock with the surface of the tissue, or other cohesive/adhesive phenomena.
As a practical consequence of this concern for traction, clamps have been applied to conduits such as vessels, and closed with a force sufficient to occlude the vessel. Where slippage has occurred, the tendency has been to increase the clamping force. With reference to the foregoing formula for friction, this increases the normal force N thereby increasing the friction or traction force F. Unfortunately, increases in the normal force N are not required for occlusion, which is the primary purpose of the clamp. Furthermore, high normal forces can create damage to a vessel, particularly the fragile endothelial lining of the vessel. What has been required for these surgical instruments is a structure which can provide a significant traction force without damage to the conduit or vessel.
In accordance with-the present invention, various structures are proposed for increasing the traction force without significantly changing the normal or occlusive force. In some cases, the traction force will be greater than the occlusive force, a condition that will be particularly appreciated for some instruments.
The surfaces providing increased traction will be advantageous in clips and clamps where there are opposing jaws which develop the normal force. Whether the improved traction is provided along one or both of the jaws can be a matter of choice.
The structures providing increased traction will also be applicable to spreaders where traction is appreciated on outwardly facing surfaces of opposing jaw members. Increased traction can be provided in the form of inserts for the jaws of clips, clamps, and retractors, or may take the form of webs providing a significant area of contact for the stabilization of organs. In the latter device, the normal force would be developed not between opposing jaws but relative to some other stationary stricture. For example, a stabilizer might be clamped to the sternum of the patient, or some other skeletal element, in order to provide a traction force against a beating heart in a bypass surgery.